In an intensive care unit, 14 newborns, without antibiotic intake, received orally 1 ml of culture broth from an antibiotic-sensitive strain of Escherichia coli marked with resistance to sodium azide. The purpose was to study the fate of the E. coli administered and to determine whether an interaction was obtained between that strain and antibiotic-resistant enterobacteria in the gut flora. In 3 infants the strain administered developed and was the only enterobacterium excreted during the first week; in 4 others the strain was recovered during one week or more, in quantities greater than 10(6)/g of stools. In 6 others, the strain appeared for less than one week, and/or in quantity less than 10(6)/g of stools. In 1 child the strain could not be recovered at all. These data show what is obtained in "holoxenic" newborns in spite of challenge with many other bacteria. They have been obtained with an ubiquitus E. coli and can be used as a control for further studies.